OCEANIC AERIAL CUEEENTS. 21; 



ing as the slopes of the islands are exposed to the moist or dry winds. Thus while 

 certain valleys enjoy a copious and even an excessive annual supply of moisture, 

 certain low-lying islands in the neighbourhood of the equator receive scarcely a 

 single shower except at long intervals. 



West and south-west of the Polynesian islands properly so called, the vicinity 

 of the great insular masses, such as New Guinea, Australia, Celebes, and Borneo, 

 attracts the aerial currents more powerfully in the hot seasons than at other times. 

 Hence are developed here, not merely gentle breezes, as in Eastern Oceania, but 

 regular monsoons of longer or shorter duration, according to the diverse conditions 

 of the environment, the extent of dry land, the altitude of the highlands, the super- 

 ficial area of spaces destitute of vegetation. In these regions the south-east trades 

 prevail during the winter season of the southern hemisphere ; but in summer the 

 normal currents set from the west or north-west, and are usually accompanied by 

 moisture -bearing clouds and heavy downpours. Thus the normal meteorological 

 system is regulated by two uniformly alternating currents setting in contrary 

 directions, and of essentially different character, one bringing fair the other foul 

 weather. Nevertheless, the endless intricacies of creeks, bays, inlets, straits, and 

 channels cause numerous irregularities and local breezes, b}' which the whole 

 system is in many places greatly modified. 



On the very verge of the range of the monsoons the atmospheric currents are 

 deflected from their regular path. The waters of Torres Strait between New 

 Guinea and Australia, obstructed by innumerable shoals and reefs, and averaging 

 not more than some 10 or 12 fathoms in depth, are heated hj the tropical suns to 

 a much higher degree than the deep oceanic basins to the east and west. The 

 consequence is a considerable increase of temperature in the circumambient 

 atmosphere, which thus becomes a focus of attraction for all the surrounding- 

 currents. The north-east trades veer round so as to set directly up the strait, where 

 they blow with great violence during the winter months. On the other hand, 

 the summer monsoons, which prevail especially in December, January, and Feb- 

 ruary, cease to set in the direction of the south-east and are deflected towards the 

 strait about Port- Moresby, thus depriving the York peninsula of its due share of 

 moisture. 



In the centre of the labyrinth of islands between New Guinea and Borneo the 

 aerial system is so disturbed by the various modifying conditions of the environ- 

 ment, that it is not always possible to determine with certainty the true character 

 of the current, whether a trade-Avind or a monsoon, or to decide to which should 

 be attributed the moisture- bearing clouds. Here the annual rainfall is generally 

 very copious, in some islands, such as Sumatra, exceeding 160 inches. The aver- 

 age temperature (78° to 82° F., according to the aspect of the seaboard) is also 

 higher than in the South Sea Islands ; it is also more uniform, varying not more 

 than four or five degrees between the hottest and coldest months. The yearly 

 range is, in fact, less than the daily variation between the morning and afternoon. 

 Owing to this equable regime the Eastern Archipelago has been called the 

 " hothouse " of the great terrestrial botanical garden. 



